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I'm currently following this guide on making an Arduino OpenTherm Controller. After finding almost all of the parts without any issue; my only question remained with the "PNP transistor" named in the hardware list. As someone who is new to electronics, this did not mean much to me, so I sought out an exact name for this transistor. Elsewhere on the webpage I found this image:

layout diagram of circuit

As you can see, it names the transistor as a BC558A. However, after searching online for this transistor I either found few or no results. On the other hand, I found many BC558B transistors like this one so I ordered a pack. After initially thinking that because no specific "PNP transistor" was named; any sort would do, I began to reconsider this which is why I am asking for help.

My question is: Will the BC558B transistor I have ordered function in the same way in the circuit? If not, what would a reasonable alternative to the BC558A transistor be, seeing as it is very hard to find online?

Thank you in advance for any help.

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    \$\begingroup\$ you have to compare datasheets to choose the references of transistors compatibles with the schematic ... \$\endgroup\$
    – user201301
    Nov 11, 2018 at 20:36
  • \$\begingroup\$ @francoisP Thanks for your response. As someone who doesn't have much experience with electronics as I am beginning to learn, I am unsure exactly what I would be looking for in the respective data sheets. Would you be able to shed any more light? Thanks \$\endgroup\$
    – Rocco
    Nov 11, 2018 at 20:41
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    \$\begingroup\$ The (null)/A/B/C suffix usually indicates a "better" grade of the device. In the case of the BC588 it indicates a higher DC gain value- though even that varies significantly even within a particular grade of the device. Without a schematic and a description of the intended function of the circuit, that's about all. If the circuit is designed well, it should minimize any effects due to such variations. \$\endgroup\$
    – user201365
    Nov 11, 2018 at 20:41
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    \$\begingroup\$ Whether it's a "better" part depends on the application and circuit design, but the B grade would have "better" performance than the A grade in terms of the hFE parameter. Usually that's a good thing, but again it depends on the rest of the circuit. You may find it works great at room temperature and fails in a hot environment (due to too much gain). Without the schematic and design intent that's about all that can be said. You can certainly put the device in and see what happens, it shouldn't harm the circuit itself, but if it controls other equipment/devices you may experience problems. \$\endgroup\$
    – user201365
    Nov 11, 2018 at 20:59
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    \$\begingroup\$ It would be rare to find a well-designed circuit that relies on a low value of hFE. The reason for the A/B/C/ grades is that in commercial quantities (i.e. 1000+ parts!) A is the cheapest, and C the most expensive, though the price differences may be small (e.g. one or two cents). Vendors who sell in small quantities often only stock the most popular grades, which is why you only found B. \$\endgroup\$
    – alephzero
    Nov 12, 2018 at 1:21

2 Answers 2

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tl;dr For this circuit, it doesn't appear to matter at all. For a precision analog circuit, it may matter if the circuit is dependent on the beta/hFE parameter.

For the BC558 transistor in particular, the datasheet indicates the following behavior for the Hfe parameter:

datasheet screenshot, showing different Hfe values for -A and -B series of this transistor

This parameter relates the collector current to the base current as a ratio. Parts labeled BC558A are guaranteed to have Hfe within one particular range; parts labeled BC558B are guaranteed to have Hfe in a different range. Beyond that, there are no significant differences--they don't have different shelf life, and all of the other datasheet parameters will still hold across these series.

However, based on the circuit shown in your question, this parameter is not strongly relevant; it appears that the transistor is being driven fully-on or fully-off and as a result this parameter has only a weak effect on the circuit's behavior. Moreover, the transistor is attached to optoisolators that themselves have very imprecise characteristics unless binned in the same way.

However, if you are trying to construct an analog circuit where this parameter is important, you may need to select a particular series of this transistor (A, B, or C) and remain within that series. Even then, many analog topologies are designed to be insensitive to certain transistor parameters to avoid this issue.

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  • \$\begingroup\$ Thank you for your detailed answer. I am unaware as to exactly what the above circuit is classified as; i.e. I cannot say for certain whether or not it is a 'precision analog circuit' unless that is something you can easily tell that I can't. All I know is that it acts as a sort of 'adapter' between the output of an Arduino and the OpenTherm protocol. From what you can see, would you say that you are almost certain it should function the same for this specific application? Thanks once again \$\endgroup\$
    – Rocco
    Nov 11, 2018 at 21:04
  • \$\begingroup\$ @Rocco I covered this in the second-to-last paragraph; your circuit in particular appears to turn the transistor fully-on and fully-off, and involves other imprecise parts. Per my best judgment, the series and Hfe does not matter for this exact circuit. However, I did mention that it might for a precise circuit for completeness' sake, and in case in the future you start working with precision analog circuits. \$\endgroup\$
    – nanofarad
    Nov 11, 2018 at 21:06
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    \$\begingroup\$ @Rocco If something is built on a protoboard, it's very definitely not precision analog circuitry. \$\endgroup\$
    – Hearth
    Nov 11, 2018 at 21:13
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if it just says PNP transistor without naming any particular one, it's likely just about any reasonably-selected one will do. You could use a BC558, a 2N3906, you might even get away with a TIP42 if it's not too picky about low β.

Like @isdi said in the comments, a suffix on the end of a part number usually just indicates a better product than the original, but one that is otherwise interchangeable with the original. Think of it like a "version 1.1" of the device.

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  • \$\begingroup\$ Thanks for your reply, just to confirm, you think that the BC558B I ordered should be okay to use instead of the BC558A? Is there any way I could tell for certain that this is the case? \$\endgroup\$
    – Rocco
    Nov 11, 2018 at 20:40
  • \$\begingroup\$ I can tell you right now that it will 100% work. BC558 is the part number; the A or B on the end can safely be ignored. \$\endgroup\$
    – Hearth
    Nov 11, 2018 at 20:42
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    \$\begingroup\$ In the case of the BC558, it appears that the suffix is not a revision; rather the datasheet indicates that parts are binned by their hfe; if this figure is significant for an application then it might be an issue (although I don't think it would be the case here) \$\endgroup\$
    – nanofarad
    Nov 11, 2018 at 20:46
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    \$\begingroup\$ @AndreyAkhmetov Right, I was using the versioning analogy as only an analogy. I can't think of any better way to put it; if you have an improvement in mind though, please do edit the answer! \$\endgroup\$
    – Hearth
    Nov 11, 2018 at 20:48
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    \$\begingroup\$ No. Parts are tested in the factory on performance, then put into "excellent" and "ok" bins (sometimes there are more bins, especially for CPUs and LEDs.). These are sold at different prices. For a BC558, the difference may be $0.01. Still much. Obsolete parts have that written on their datasheets. (Or, you cannot find a recent datasheet any more.) \$\endgroup\$
    – Janka
    Nov 11, 2018 at 21:06

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